Apparatus for automatic voltage and current control



March 16, 1954 P. E. HAGGERTY ET AL APPARATUS FOR AUTOMATIC VOLTAGE AND CURRENT CONTROL 2 Sheets-Sheet 1 Filed Aug. 31, 1948 J8 AAAAAlAAAAAl vvvvvvvvvvvv March 16, 1954 HAGGERTY ET AL 2,672,527

APPARATUS FOR AUTOMATIC VOLTAGE ANO CURRENT CONTROL Filed Aug. 31, 1948 2 Sheets-Sheet- 2 Patented Mar. 16, 1954 UNITED STATES PATENT OFFICE 2,672,527 APPARATUS FOR AUTOMATIC voLTAGE AND CURRENT CONTROL Patrick analg sia and Robert w. Olson, Dallas,

Texi, assigriors to Texas Instruments Incor orated,*a corperatio'n of Delaware Application August 31, 194 8, SerialNo. 47,014

screams; (or. 179-471) This invention relates to autcmaticamplitude control for electrical currents and voltages and more particularly to automatic volume or amplitude controls for electrical amplifiers such as are commonly used in connection with seismic surveying.

A common method of automatically controlling amplitude in seismograph amplifiers, and volume in radio amplifiers, consists in rectifying a portion ofthe amplified signal current, using this rectified current to develop a potential across a resistance, and applying this potential through suitable filtering, as a bias to one or more amplifier tubes. I I, p

This method has several disadvantages. First, it is diflicult by this method to obtain a large enough. change-in grid bias for a given change in signal to effect a very close control of amplitude. Ordinarily the control will be close enough for radio amplifiers, but not close enough for scientific measuring devices suchas seismograph amplifiers. Second, there is a tendency to feed back. the fundamental. signal frequencies into the grids of the preceding amplifier tubes thereby causing regeneration, degeneration, ordistortion, depending upon the phase relationship.

It is the purpose of" this invention to provide an automatic amplitude control for electrical currents and voltages, that avoids these difliculties and is particularly adpted to the controlling of seismegraph amplifiers, but adapt able to use in numerous other circuits in which generally similar control is desirable;

Broadly stated. the present invention com prises varying the conductance of a thermionic tube by varying its cathode heating current in accordance. with the-amplitude of the voltage or current to be controlled and usingthis variation in conductance to effect the control, I A,

If the plate current of a thermionic tube is plotted graphically against-its filament or heater current, it will be found that as the heater current is increased, the plate current rises, at first very; slowly, then with extreme rapidityand thereafter very-slowly again until sufficient current is passed through the filamentor heater to cause it to burn out. The control contemplated by" this invention maybe accon'iplished by operating the thermioniccontrol tube about either the upper or lower bend of its filament-currentplate current characteristic curve, or on the s'teep portion of the curve. I 4

The thermionic control tube may be ofpractically any type,but eioaesiiaviug directly heated cathodes of relatively low current rating are gen- "erally preferred. Triodes, tetrb'de's, pentodes, etc.,'inay be used,v and dual purpose tubes may also be used, but generally are unnecessary. Indirectly heated cathodes may also be used in some instances but generally are too slow in their response to changes in heater current to be, effective. I:

v In general, the filament of the control tube may be heated either directly or indirectly by all er part of the signal current from the amplifier or by all or part of the current that is to be controlled. Since the control tube acts as a full wave rectifier, with respect to the filament current, it automatically eliminates the fundamental frequency of the signal current from its plate I current and the temperature lag in the filament generally tends to eliminate all except very low frequencies. v

A plate circuit return for the control tube may be taken from one side of the filament or the filament may be bridged by resistance and the plate return attached to a center tap on that resistance to further avoid introducing signal current components into the plate circuit of the control tube. Alternatively, the filament itself may be provided with a center tap for the plate return or the transformer driving the filament may be center tapped. v

'When the current to be controlled isof a sufficiently high frequency, the filament current of the control tube may be fed to the control tube through condensers to isolate the controltube circuit from the source of the current or voltage to be controlled insofar as direct current is concerned. Alternatively, the filament current may be supplied through a transformer for the same purpose. r v v n The control tube is now a variable resistance havinga wide variation in resistance in response to moderate changes in filament current. Furthermore, this variation in resistance does not occur linearly with respect to changes in filament current and this fact makes the variable re sistance of the control tube particularly useful as a means to control electrical currents or volta sr.

In orderto use the variable resistance; of the control tube for the purpose of controlling currents or voltages, it is necessary that-a D. C. plate voltage be applied, in the proper direction, between the filament and plate of the control tube. Thus, the control tube may be connected into a capacitance-resistance coupled, voltage amplifier tube as a part of theload resistance and the usual plate supply voltage of the amplifier tube will be partly applied across it. Then as the signal level increases and the filament current is thereby increased in the control tube, the load resistance furnished by the control tube will drop sharply, thus reducing the voltage amplification and controlling the amplitude of the output signal.

Since the resistance across a diode is practically infinite when the cathode is cold, it is desirable in such circuits to place a high resistance between the cathode and plate of the control tube so that plate voltage will be applied to the arnpifier tube when the circuit is first energized and until sufficient signal current flows in the filament of the control tube to lower its resistance to a point where a voltage can be applied through it to the plate of the amplifier tube. Alternatively, the circuit can be such that sufiicient current will fiow through the filament of the control tube even under no-signal conditions, to lower the resistance of the control tube to some finite value that will permit voltage to be applied through it to the plate of the amplifier tube.

In other instances, it may be desired to use the control tube to control either the grid bias of one or more amplifier tubes or the attenuation of an attenuating network. By appropriate circuit arrangements the control tube can be caused to accomplish either of these purposes or to exercise its control in any other manner in which a variable resistance could be used to control the current or voltage in a circuit. The only special problem involved is that of applying the necessary direct current plate voltage to the control tube.

No attempt will be made here to detail all of the methods by which the control tube may be caused to control particular currents and voltages but an example of the preferred use of the control tube to control a practical seismograph amplifier wil be illustrated and a few additional examples will be given of the exercise of the control tube functions by the control of grid bias, plate load, and attenuation of attenuating circuits.

For further details and a more complete understanding of the principles and preferred methods of applying the principles of this invention, reference may be had to the appended drawings and the folowing detailed description thereof.

In the drawings:

Figure 1 is a schematic illustration of one method of applying the principles of this invention to the control of an amplifier, in such a way that the control tube controls the grid bias of one or more tubes of an amplifier;

Figure 2 is a schematic illustration of a method of applying the principles of this invention so that a control tube controls the operation of an amplifier by controlling the plate load of one of the tubes of an ampifier;

Figure 3 is a schematic illustration of another method of applying the principles of this invention so that the control tube controls the degree of attenuation effected by an attenuating network; and

Figure 4 is an illustration of the use of the principles of this invention to control the operation of a seismograph amplifier.

As is illustrated in Figure 1, the principles of this invention have been applied to the control of a typical resistance-capacity coupled vo tage amplifier stage. The input to this stage is applied betwem a B- connection 1.; and a grid conn .ction II in the usual way with a condenser i2 located in the grid connection to block direct current. From the blocking condenser i2 connection is made directly to the control grid 13 of a conventional triode amplifier tube It of the indirectly heated cathode type. Obviously, 3. directly heated cathode type of tube, a tetrode, pentode or other special type of tube could be used in place of the triode 14 that has been illustrated.

The cathode 15 of the amplifier tube is shown as connected through a resistor l6 and a condenser I! to the 8- line It and these parts serve to provide a normal grid bias through a resistor l8 one end of which is connected to the B line and the other end of which is connected by resistance ill to the grid ill. The plate circuit of this tube is conventional, the plate 26 being connected through a resistor 2i and a B battery 22 which furnishes plate potential, to the 3-- line H]. A condenser 23 is shunted. across the B battery 22 in accordance with common practice. The plate 26 is connected to the grid of the next amplifier tube or to the detector or other device to which the amplifier signal is to be supplied, through a blocking condenser 24 in the usual manner. Thus far we have a conventional voltage amplifier circuit of the resistance-capacity coupled type.

In order to app'y the principles of this invention to the circuit thus far described, the filament 25 of a diode tube 26 is connected between the B line it and a source of control current (not shown) Ordinarily, this source of control current will be the ungrounded side of the out put of the amplifier or the output of some stage of the amplifier. Quite often the output will exist between the B line and some other point in the circuit from which it can be taken directly, but if not it can be applied between the B line it) and the filament lead 27 of the diode 26 through the medium of a transformer or condensers. Whatever attenuation is found to be necessary can be placed in the filament lead 2'! to reduce the current flow therethrough to the amount required to operate the diode 26 at the desired point in its characteristic curve.

The plate 28 of the diode 2B is then connected through a source of voltage such as a battery 29 to the resistance 19 which connects to the grid of the amplifier triode M. With this circuit arrangement, as long as the filament 25 of the diode 28 is cold, the circuit which includes the diode and the battery 29 will be open and no current will fiow through this circuit and hence through the resistor 18. Therefore, no voltage will be developed across this resistor. As the filament 25 of the diode is heated an increasing amount of current will pass through the diode 26 and through the resistance 18 thus developing an increasing potential across this resistance. By reason of the fact that the negative terminal of the battery is connected to the grid end of the resistor Hi this voltage will be negative and will tend to reduce the amplification of the triode It, thus reducing the overall amplification of the amplifier until the current to the filament 25 of the diode 25 is reduced, which in turn reduces the current through the resistor I8 and reduces the negative bias on the grid iii of the triode [4. By arranging the circuit to operate the diode 26 at the lower bend of its characteristic curve or on the steep portion of its characteristic curve, a very exact control can easily be achieved. By reason of the filtering accomplished by the diode 26, the fundamental frequency of a control signalwill notbe fed back into the grid circuit. of the amplifier tube and by reason of the temperature. lag of the filament there is little. tendency to feed back any except very low frequencies into the grid of the amplifier tube 14. Thus relatively little or no filtering. is required between the outputv of the amplifier and the. filament of the diode 26.

. In F gure 2 there is illustrated anapplicationof the principles of this. invention to the control of an. amplifier of the same. general type, by changing he plate load. of the-voltage amplifier tube involved. Again. the input is to the 3- line It) and a grid lead ll which contains ablocki s condenser l2. In this case, however, the, grid is also connected directly to the 3* line through a grid resistor 3.0.. Normal bias is again provided by connecting the cathode to the 13 line through a. resi tor is and a. condenser.- .II. The plate 2 0 of the triode I4 is again connected through a resistor 2! and a B battery 22 to the B- line In and this battery is again shuntedv by a condenser 23, all as before. A ain the plate is (ionnected h h a blocking condenser 24. to the grid of the next amplifier tube.

In the arrangement'shown in Figure 2,. however, control of the voltage amplific ion of the amplifier tube is provided by connecting a diode tube 3| across the plate resistor 2|. In order that the source of plate voltage, the battery 22, also supply plate voltage for the diode, plate 32 of the diode is connected to the battery end of the plate resistor 21, that is to a point between the. resistor 2| and the positive terminal of the battery 22.. The filament 33 of the diode Si is then connected to the opposite end of the resister 21.. In order to isolate the plate circuit of the triode M from the source of control current, control current is supplied to the filament 33 of the diode 3i through a, transformer 34. By using .a center tapped. filament 33 in the diode 3i and connecting the center tap to the plate lead of the triode l4 and supplying current to the two outer leads of the filament 33 from the transformer 34, the feeding of signal currents, into the plate circuit of the amplifier tube is avoided.

The. result of this circuit is the shunting of the plate resistor by the diode 31. As the i nal current at some selected stage in the amplifier becomes excessive, a sufficient amount of that signal current is fed back into the filament 33 or the diode 3! to cause that diode to become conductive. This shunts the. plate resistor 2i greatly reducing its eifective resistance and hence reducing the plate load on the triode l4 and thereby reducing the voltage amplification of this stage of the amplifier.

Still another embodiment of this invention is illustrated in Figure 3. in which the principles of this invention are applied to control theattenuation of an attenuating circuit. The attenuating circuit receives its input current through two leads and 4! and supplies current, attenuated to the desired extent through leads 42 and t3. In this circuit lead 40 is connected to lead 42 through a resistance 44, and lead 4} is connected to lead 43 through a resistance 45. A shuntin resistor 46 is then connected between leads 4t and 42 and another shunting resistor 41 is con nected between lead 40 and lead 43. If resistances M, 45, 46 and 41 are properly related, a balanced bridge will result with infinite attenuation between points 40 and 4| and points 42 and 43.

Changing the effectiveresistance of one of the resistors in this attenuating network. such for example as the resistance of the. resistor 45. will vary the attenuation over a wide range without. otherwise greatly affecting the characteristics of the circuit.

It would be difl'icult. to change the resistance of the resistor 45 by shunting it with a. single con.- trol diode and. therefore, a dual diode has been used in this embodiment. This dual diode 50 contains a single filament 5| and two plates 52 and 53'. By connecting one of these plates to each end of the resistor 45 and supplying plate voltage to amid point. on the resistor 45 from a source of plate supply such as battery 54, the ends of the resistor 45 may be maintained at-the same potential and yet the two halves of the diode effectively shun-ted across the resistor 45. The filament of the diode 50 may either be made with a center tap return or may be bridged, as shown in the drawing, by a resistor 55 or driven by a center-tapped transformer, so that the B- battery return may be connected to a central point. Thus the possibility of introducing signal currents into the attenuating network may be lessened. As before thesignalcurrents which are to control the attenuation are applied to the filament of the diode, in this case through leads 56 and 51.

In Figure 4, there is illustrated a rathercomplete arrangement for the use of the principles of this invention in the seismograph amplifier circuit. In this circuit, electrical signals generated by a seismometer in the course of making a seismic survey are introduced into the pri 'mary of a transformer 60 through leads 6| and 62. The opposite ends of the secondary of this transformer are connected to the control grids of two remote cut off 'pentodes 63 and 64 connected as triodes so that these tubes will act in push-pull to amplify the signals. A grid return line 65 is connected to a center tap on the secondary of the transformer 60. a

A varistor 66-, which is a non-linear resiste ance with strong limiting action, is connected between thetwo control grids of the two tubes 53 and 64 to provide a preset amount of signal limiting of the incoming signals.

The cathodes of the tubes tit-and 64 are connected through a conventional cathode resistor 61 to a cathode return line 68 whichcon-nects to the plate supply and receives a voltage that is negative with respect tothe platevoltages of the amplifier tubes but positive with respect to ground. This plate supply is not shown but two terminals 69 and in are shownand a-n appropriate source of voltage is to be connected thereto. Usually the -volt terminal of the plate supply will be attached to terminal 69 and the -vol't terminal of plate supply attached to terminal 10. The negative terminal will then be connected to ground. A second: pair of'tubes, constituting a second stage of amplificationmay be similarly connected for control.

In the two tubes 83 and 64:. the scr n rids of the variable mu or remote cut off pentodes are used as plates and. are supplied with plate voltage. through plate load resistors H and 1.2, respectively. The suppressor grids and plates of these tubes are not used for amplification but grounded for shielding. The result is a pushpull remote cut off triode stage. This. is. not an essential part of this invention.

Signal currents from the two tubes. 63- and G4 are introduced into the primary of ,a coupling transformer 13 which is connected between the screen grids (acting as plates) of the two pentode tubes in series with a blocking condenser 14.

From the secondary of the coupling transformer 13, the amplified signal currents are again impressed upon the control grids of another pair of pentode tubes connected exactly as are the first pair of pentode tubes and again the signal output is introduced into a coupling transformer I5. As illustrated, the amplification of the second pair of tubes is also controlled. From the coupling transformer 15 the signals are introduced into a conventional amplifier and the output of this amplifier is carried by connections I6 and H to a conventional recording galvanometer.

Control of the amplification of this circuit is provided by a relay and a control circuit. The relay receives actuating power through connections '18 and 19 from the direct current output of the amplifier. Thus when the first seismic waves pass through the amplifier, current is caused to fiow in lines 18 and I9. These lines 18 and i9 are connected to a relay 30 which operates to open a switch 8| thus actuating the control circuit in a manner that will now be described.

When closed, the switch 8i serves to connect the grid return lines for the two pairs of amplifier tubes to the center tap of a potentiometer 82. One end of this potentiometer is grounded. The other end is connected to the cathode return line 68 and this line is normally maintained about 90 volts above ground potential so there is a potential drop of around 90 volts across the potentiometer 82. By appropriate adjustment of the slider of the potentiometer 82, any desired potential may thus be placed on the grids of the four tubes involved, from the potential of the cathodes of these tubes to a potential around 90 volts below, or negative, with respect to the cathodes.

Normally, the potentiometer will be set so that prior to the opening of the switch Bl, the potentials on the grids will be such that the first incoming signals will be amplified to approximately the desired extent. As soon as the first incoming signals arrive and are amplified, the relay 8!) will be actuated to open the switch BI and the potential of the grids of the four tubes will thereafter depend upon potential across a condenser 89 which is connected between the grid return line 65 and the cathode return line 68.

As long as this condenser 89 retains its original charge the relative potential of the grids will remain the same as it was before the switch 81 was opened. However, a discharge path to the cathode return line is provided through a resistor 83 connected in series with a vacuum tube 84 and a second resistor 85. The vacuum tube 84 has its filament connected to the resistor 83 which in turn is connected to the grid return line 65 and has its plate connected to the resistor 85 which in turn is connected to the cathode return line 68. This provides a path through which the negative charge on the grids of the pentode tubes can be dissipated and the grids reach the same potentials as the cathodes. However, a control tube 86 is connected between the plate of the discharge tube 84 and ground so as to control the rate at which the negative potential is reduced. Filament current for the discharge tube 84 is provided by battery 87 through a resistor 88 which regulates the amount thereof. Filament current for the control tube 86 is provided from the output or the amplifier by means or a connection so and ground connections.

The operation or the control tube on results from the fact that the current passing through this tube must also pass through the resistor and the fact that the greater the current is that passes through resistor 85, the greater is the potential drop across it. Thus as long as the filament of the control tube 85 is heated to a relatively high degree and the resistance of the control tube 86 is relatively low, the potential drop across the resistor 85 will be relatively high and the potential of the plate of the discharge tube 84 will be relatively slight with reference to ground. Thus the current flow through this tube will be zero or some very small amount and the potential of the grids of the four tubes will not be reduced or at least will not be reduced rapidly while the control tube 66 is conductive. As the output of the amplifier falls due to the reception of weaker signals, the filament of control tube 86 will be heated to a less extent; the resistance of this tube will increase; the current flow through this tube and through resistor 85 will decrease; the potential drop across resistor 85 will decrease; and the potential on the plate of the discharge tube 84 will increase thus increasing the current fiow therethrough and consequently decreasing the negative bias on the grids of the four pentode tubes. This in turn will increase the amplification of the four pentode tubes and raise the output of the amplifier until the filament or the control tube 88 is again heated to such an extent that the current flow through it and hence the potential drop across the resistor 85 will have increased suinciently to stop, or at least substantially stop, the flow of current through the discharge tube 84 and hence the discharging of the condenser 89 which supplies the bias to the grids of the tubes. This cycle of operations will continue during the reception of a whole series of seismic wave signals and will tend to stabilize and control the amplification at a desired level during the reception of the whole series of seismic waves despite the fact that they are generally of progressively decreasing amplitude. Obviously, since the discharge tube 34 acts as a rectifier, the grid bias can never be increased except by closing the relay switch 8|.

Both the discharge tube 84 and the control tube 85 have been shown in the drawing as tetrodes because this type was used in the original work on this amplifier. However, diodes, triodes,'

tetrodes, pentodes or multi-functional tubes could be used in their stead. Similarly, other types of tubes than pentodes could be used in the push-pull amplification stages.

It is to be understood that suitable attenuation will be provided in the circuits leading from the amplifier to the relay and from the amplifier to the control tube 85 to cause these parts of the circuit to operate in the desired manner and to prevent undue dissipation of the signal into these circuits. Many other modifications and many other changes in details may obviously be made within the scope of this invention.

What is claimed is:

1. An automatic volume-controlled amplifier that comprises a pair of thermionic tubes connected in push-pull relationship, means to supply the plates of said thermionic amplifier tubes with a potential positive with respect to the cathodes, a common grid return for the control grids of said two thermionic tubes said common grid return connecting to said cathodes through a condenser, a resistor connected at one end to said cathodes, means to supply the opposite end of said resistor with a potential negative with respect to said cathodes, a variable contact on said resistance, a magnetically-operated switch for connecting said variable contact to said grid return, connections connecting said magnetic, switch to the output of said amplifier so that said magnetic switch will be opened upon the passing of the first signals through the amplifier and will thereafter remain open, a thermionic control tube having its cathode connected to the negative end of the above-mentioned resistor and having its plate connected through a second resistor to said cathodes, connections between the heating element of said control tube and the output of said amplifier for heating said heating element in accordance with the output of said amplifier, and a rectifier and resistance connected in series between the plate of said control tube and said grid return for permitting the discharge of the above-mentioned condenser under the control of said control tube.

2. In an amplifier circuit that comprises at least one thermionic tube amplifier, connections for applying signals to be amplified between the grid and the cathode thereof, means for supplying operating grid bias to the grid, a plate load, means for taking amplified signals from the plate 10 circuit and means for applying operating plate voltage between the plate and cathode, the improvement that comprises an automatic volume control for said circuit, including a thermionic control tube, connections between the output circuit oi the amplifier and the heating element of the control tube for supplying a heating current to the heating element proportional to the output of the amplifier, and connections connecting the plate circuit of the control tube into the grid circuit of the amplifier tube, said connections consisting of a resistance connected into the gridcircuit of the amplifier tube and further connections connecting the plate circuit of the control tube in series with a source of voltage and across said resistor, the control tube and source of voltage being so connected as to apply negative voltage to the grid when the control tube is conductive, so that increases in the output of the amplifier decrease the resistance through the control tube and lower the potential of the grid of the amplifier tube and thus reduce the amplification of the amplifier tube.

PATRICK E. HAGGERTY. ROBERT W. OLSON.

References Cited in the file of this patent UNITED STATES PATENTS Number 

